Surface properties are of critical importance to a broad range of materials and devices. A number of surface functionalization techniques—thiol-gold self-assembled monolayers, chloro- or methoxy-silane surface attachment, carboxylic acid esterification, and phosphonic acid deposition—have produced materials leading to entire fields of research, and have significantly advanced others. However, the ability to rapidly and reproducibly create organic monolayers using gas-phase deposition to produce strong uni- and/or bi-dentate surface bonding on highly porous oxide materials that, when desired, are also able to efficiently transfer electrons to and from the substrate has yet to be realized.
A self-assembled monolayer consists of a single layer of molecules on a substrate. The formation of self-assembled monolayers has historically been accomplished through gold-alkylthiolate self-assembling monolayers; chlorosilane or alkoxysilane surface attachment; carboxylic acid esterification; and phosphonic acid deposition.
These methods form the self-assembled monolayer through the formation of chemical linkages between the reactant and the substrate. Gold-alkylthiolate self-assembling monolayers, such as shown in FIG. 1A, function through their ability to form a gold-thiol-alkyl linkage. Chlorosilane or alkoxysilane surface attachment to a metal oxide, such as shown in FIG. 1B, functions through the formation of a metal-oxygen-silicon linkage. Carboxylic acid esterification, such as shown in FIG. 1C, functions through the formation of a metal-ester linkage. Phosphonic acid deposition, such as shown in FIG. 1D, functions through the formation of not only a metal-oxygen-phosphate linkage but also a weak hydrogen bond between the phosphonic acid derivative and the oxide surface.
To date, the best-studied monolayer deposition technique, with resulting materials characterization, is the gold-alkane thiolate self-assembled monolayer. Self-assembled monolayers produced from gold-alkylthiolate attachment are an important element of materials used in such fields as nanoscience and nanotechnology.
Surface functionalization of oxide materials, with silanization the most common, has been utilized in diverse fields, such as separation science, catalysis, sensing, optics and tribology. Although silanization has multiple advantages for surface functionalization, including ease of surface attachment of the chloro or alkoxysilane functional group to surface hydroxyl groups, the disadvantages of silanization include incomplete functionalization of surface hydroxyls, difficulty in controlling monolayer formation, long deposition times, and inefficient electron transfer from the monolayers produced.
Carboxylic acid esterification and phosphonic acid deposition have helped establish some fields of study in the area of optoelectronics and associated industries. For example, the majority of dye-sensitized solar cells use carboxylic acid esterification to modify the surface of nanoporous oxide materials through a carboxylic acid anchoring group. Although carboxylic acid surface deposition has multiple advantages, including excellent electron transfer and ease in synthesizing materials containing this functionality, it can be difficult to deposit uniform monolayers of carboxylic acid containing molecules. The disadvantage of phosphonic acid deposition is the difficulty in synthesizing the phosphonic acid derivatives as well as lower electron transfer efficiency of the resulting hybrid materials, but this type of deposition does offer advantages of robust bonding that is more hydrolytically and thermally stable than monolayers formed from organosilanes.
Considering the state-of-the-art, a rapid and easy surface functionalization technique capable of producing strong chemical bonding of self-assembled monolayer hybrid materials possessing efficient electron transfer between the substrate and self-assembled monolayer is highly desired and its development would be a tremendous achievement for multiple fields of study. For example, the field of optoelectronics and separation science would benefit through rapid and efficient monolayer formation and increased active surface coverage of the organic material on the inorganic oxide substrate, thereby increasing the efficiency of optoelectronic devices and separations, respectively.
The current methods of depositing self-assembled monolayers onto inorganic oxides are time and material intensive. These methods typically involve refluxing or dip-coating the substrate in organic solvents containing high concentrations of the deposition material for periods of 4-18 hours, and even up to 24 hours or more. Still, these methods result in only partial surface modification, limited surface passivation (i.e., incomplete reaction of active surface sites, such as surface hydroxyls, with monolayer material such that a large fraction of the active surface sites are unreacted), and/or segments of the surface where the reactant has deposited onto itself (i.e., aggregation) forming a non-functioning multilayer. Such processes also constitute up to about 80% of the fabrication time of dye-sensitized solar cells (DSCs), leads to significant dye waste, and necessitates the use of toxic and difficult to dispose of organic solvents.
Further, with the improvements in DSC technology since the modern DSC design being introduced in 1991, including breakthroughs in alternative dyes, photoelectrodes, and electrolytes, efficiencies of laboratory-based DSC modules have only increased to about 12.3% over the last twenty years, including a minimal increase over the last 15 years. Although future work may uncover technology to significantly increase the efficiency of DSCs, there is a need to reduce the power generation costs in order to increase the feasibility of DSCs as an alternative energy generation technology.
Accordingly, there is a need in the industry for efficient, cost-effective, and sustainable materials and methods wherein a self-assembled monolayer is bonded to a metal oxide, wherein these materials are of high quality, durable, and provide surface passivation that promotes efficient electron transfer.